There is a fine line between symbiosis (mutualism) and pathogenesis. Most, if not all, animals are colonized by bacteria, many of which-the organism's "normal flora"-are tolerated or even encouraged. To establish a symbiotic relationship with an animal, a prokaryotic organism must be able to recognize and respond to a specific environment, establish communication with the host, colonize the host tissue, evade defense mechanisms, persist and multiply. Each of these steps is mirrored in pathogenic interactions, whether primary or opportunistic-and even normal flora have pathogenic potential in the event of excessive multiplication or introduction into areas they do not ordinarily inhabit. Despite the ubiquity of bacterium-animal symbioses, little is known about the genetic and molecular details of these interactions. In addition to increasing basic understanding of these important prokaryote-eukaryote alliances, the study of symbiotic interactions is expected to improve understanding of how pathogenic organisms adopt, subvert, or bypass the mechanisms leading to symbiosis and why they provoke dissimilar responses from the eukaryotic cell. The symbiosis between the bioluminescent marine bacterium Vibrio fischeri and its host, the squid Euprymna scolopes, provides an excellent opportunity to study the biology of a bacterium which can form a long- term, mutualistic association with an animal host. The symbiosis has been well-characterized at the organismal level. We now propose to begin the detailed molecular characterization of the interactions between these organisms, including establishment of the symbiotic state, signalling between the two organisms and developmental changes occurring in both partners as the symbiosis progresses. The proposed research will investigate the requirements for V fischeri colonization of host tissue, using a straightforward screen for transposon-insertion mutants defective in the initial symbiotic events. Identification and characterization of the critical colonization genes will lay the foundation for understanding signal exchange between a prokaryote and a eukaryote during the establishment of a long-term association.